Merge branch 'docs/notice-about-gitlab-runner' into 'master'

Add notice about GitLab Runner to requirements docs

This is related to #14589, and problems that may stem from running
GitLab Runner on same machine user installed GitLab web app on.

See merge request !3518

doc/ci/runners/README.md

@@ -7,6 +7,10 @@ through the coordinator API of GitLab CI.
 A runner can be specific to a certain project or serve any project
 in GitLab CI. A runner that serves all projects is called a shared runner.
 
+Ideally, GitLab Runner should not be installed on the same machine as GitLab.
+Read the [requirements documentation](../../install/requirements.md#gitlab-runner)
+for more information.
+
 ## Shared vs. Specific Runners
 
 A runner that is specific only runs for the specified project. A shared runner
@@ -140,7 +144,7 @@ to it. This means that if you have shared runners setup for a project and
 someone forks that project, the shared runners will also serve jobs of this
 project.
 
-# Attack vectors in runners
+## Attack vectors in Runners
 
 Mentioned briefly earlier, but the following things of runners can be exploited.
 We're always looking for contributions that can mitigate these [Security Considerations](https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/security/index.md).

doc/install/requirements.md

@@ -79,6 +79,26 @@ With less memory GitLab will give strange errors during the reconfigure run and
 
 Notice: The 25 workers of Sidekiq will show up as separate processes in your process overview (such as top or htop) but they share the same RAM allocation since Sidekiq is a multithreaded application. Please see the section below about Unicorn workers for information about many you need of those.
 
+## Gitlab Runner
+
+We strongly advise against installing GitLab Runner on the same machine you plan
+to install GitLab on. Depending on how you decide to configure GitLab Runner and
+what tools you use to exercise your application in the CI environment, GitLab
+Runner can consume significant amount of available memory.
+
+Memory consumption calculations, that are available above, will not be valid if
+you decide to run GitLab Runner and the GitLab Rails application on the same
+machine.
+
+It is also not safe to install everything on a single machine, because of the
+[security reasons] - especially when you plan to use shell executor with GitLab
+Runner.
+
+We recommend using a separate machine for each GitLab Runner, if you plan to
+use the CI features.
+
+[security reasons]: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/security/index.md
+
 ## Unicorn Workers
 
 It's possible to increase the amount of unicorn workers and this will usually help for to reduce the response time of the applications and increase the ability to handle parallel requests.